This invention relates to an ultrasonic Doppler imaging apparatus and method that utilize a different type of transmit signal to enhance the resulting Doppler image.
Color Doppler imaging is a known ultrasonic imaging mode that is particularly used in medical imaging applications. In color Doppler imaging, a transmit signal is applied to a transducer array to create ultrasonic beams in the tissue of interest. Ultrasonic echoes resulting from these beams are received by the transducer array and converted to receive signals that are beamformed and then applied to a color Doppler processor. The color Doppler processor can use autocorrelation techniques to estimate various Doppler parameters such as energy, variance and velocity for Doppler shifted echo signals associated with moving targets. Another type of motion processor performs frame-to-frame correlations to determine direction and magnitude of scatterer motion. Typically, the resulting Doppler parameters are displayed using a color display, in which differing levels of Doppler energy, variance or velocity are color-coded. For example, Doppler ultrasonic imaging can be used to diagnose blood velocity in cardiac imaging applications.
Another type of Doppler imaging is termed Doppler tissue imaging, and is described for example in Arenson, et al. U.S. Pat. No. 5,285,788, assigned to the assignee of this invention. In this imaging mode, low pass filters and clutter filters are reduced or eliminated to provide Doppler images of slowly moving or stationary tissue.
All of these Doppler imaging modes suffer from the disadvantage that conventional transmit pulses result in a transmit beam that varies in width with depth or range. Such depth-dependent variations can represent an important limitation, because the signal-to-noise ratio of the resulting echo signal will vary substantially as a function of depth. Since the transmit beam is not optimally uniform in width, the use of multiple receive beams for a single transmit beam may be limited due to non-uniformity of insonification intensity and beam width as a function of depth near the edge of the receive beams.
The present invention is directed to an improved ultrasonic Doppler imaging system that reduces or overcomes these problems of the prior art.